Characterization of Weathering Products on Duluth Complex Rocks and Implications for Metal Mobility, Mesaba Deposit, Minnesota, USA

Abstract

The Mesaba deposit is a copper-nickel-platinum group element (Cu-Ni-PGE) prospect hosted by the Duluth Complex and located in northeastern Minnesota near the town of Babbitt. If mined, sulfide-bearing waste rock will be generated. During the environmental review process, predictions of the quality of mine-impacted waters associated with various mine features (e.g. waste rock piles, pit lakes) will be used to inform waste and water management strategies during and following mining. The quality of mine-impacted water at a future mine will reflect the combined effect of kinetically controlled release of constituents by mineral oxidation and dissolution reactions and attenuation of constituents through precipitation and related weathering product formation reactions. This study characterizes weathering products that formed on exposures of sulfide-bearing rock that occurs on the Mesaba Project site. The results from material characterization were used to inform a conceptual model that incorporates weathering products as controls on the concentrations of metals and metalloids in waters draining Duluth Complex systems. Weathered samples from the site were characterized using bulk chemical techniques, transmitted and reflected light microscopy, powder X-ray diffraction (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and synchrotron based micro-analysis including X-ray fluorescence (µXRF) and X-ray absorption spectroscopy (µXAS). Weathering product characterization showed copper can be attenuated in iron-oxide rich weathering products that show compositional banding. The banded coatings contain alternating silicon and iron rich layers and can contain up to several weight percent copper (as determined by semi-quantitative EDS measurements). In addition, the secondary copper mineral malachite was identified as a surface precipitate. Arsenic was also found to be contained in the previously mentioned coatings in the form of both As (III) and As (V). Weathering products containing other contaminants of interest (i.e. Ni, Co, Zn) were not positively identified, possibly due to the detection limits of the methods used. Following solid phase characterization, screening-level geochemical modeling using the program Geochemist’s Workbench was conducted to compare the theoretical control exerted by weathering products to actual water quality observations from weathering experiments conducted by the Minnesota Department of Natural Resources and as part of the project development. Based on composition trends, solubility controls may include malachite for copper, Ni(OH)2 for nickel, CoCO3 for cobalt, and smithsonite for zinc. In general, the theoretical controls are not definite, and the water chemistry data are highly variable. The results of this study are relevant to predicting the fate of metals released from Duluth Complex rock exposed to oxidizing atmospheric conditions such as would be found in mine features (including waste rock piles, pit lake high walls) and experimental systems (including humidity cell tests, column tests, field-based weathering experiments).